deepseek-harness/docs/rfc/implemented/feature/2026-06-30-hook-protocol-lib.md

RFC: dsh-hook-protocol — the shared Claude Code / Codex hook wire-protocol core

Status: implemented

Problem

The hooks subsystem ships two bridge plugins: one that runs a user's existing Claude Code (CC) hooks, one for Codex hooks. Studying the reference implementations (~/repos/refs/claude-code, ~/repos/refs/codex) surfaced a decisive fact: Codex deliberately reimplements a SUBSET of the CC hook protocol. Its engine reads the same hooks.json, uses the same matcher-group shape, the same exit-code/structured-stdout output contract, and the same command-hook execution model — Codex's source even names the engine after Claude's and comments where it "intentionally diverges." So the two bridges would otherwise duplicate the bulk of the protocol.

This RFC introduces @deepseek-ai/dsh-hook-protocol, a library (not a plugin — it registers and injects nothing) holding the genuinely-identical primitives both bridges build on. The split between shared and per-dialect is the design's center of gravity.

Decision

A new packages/hooks/ group with hook-protocol as a pure library. It owns four primitive families and the hook/* session events; each bridge plugin (dsh-hooks-claude, dsh-hooks-codex) owns what genuinely differs.

Shared (here):

• Matcher — matchesMatcher(pattern, query, mode). The ONE axis the dialects differ on is collapsed to the mode parameter: claude treats a pure [A-Za-z0-9_|]+ pattern as a literal (pipe = exact-match alternation) and anything else as a regex; codex is always an unanchored regex. Match-all on absent/''/'*'; an invalid regex matches nothing (never throws into the loop).
• Execution — runHook(bash, hook, options). Runs a command hook through the ctx.bash seam rather than a bespoke spawn: the executor already provides the scrubbed-but-overridable env, process-group kills, and timeout the protocol needs, and dsh-bash's stdin/env fields (added for exactly this) are the trusted-plugin surface an in-process bridge is allowed to use. It serializes the bridge-built payload to stdin (trailing newline iff CC), honors the hook's timeoutSec (else DEFAULT_HOOK_TIMEOUT_MS, the 10-minute reference default both dialects share), and never throws (an executor rejection becomes a non-blocking-error HookOutput).
• Decode — parseHookOutput(exit, stdout, stderr), the exit-code + structured-stdout codec, producing a dialect-neutral HookOutput. Exit 0 → lenient JSON parse of stdout; exit 2 → blocking error with stderr as the reason (surfaced as decision: 'block' so no caller needs a separate exit-code branch); other → non-blocking error. Parses the CC structured-stdout fields that have a consumer on some path (continue/stopReason/decision/hookSpecificOutput.{permissionDecision,additionalContext,updatedInput}/systemMessage); the bridge honors only the subset meaningful for its dialect. Fields with no consumer on any path are not parsed at all (CC's suppressOutput — hook stdout never enters a transcript here, so there is nothing to suppress; see the tighten-hook-protocol-contract RFC).
• Merge — mergeHookOutputs(outputs), folding multiple matched hooks into one most-restrictive MergedHookOutcome: permission precedence deny > ask > allow, halt sticky on the first continue:false, block reasons joined \n\n, context/system-messages accumulated in order.
• hook/* session events — hook/invoked / hook/result, declaration-merged into SessionEventMap (log-only, like compact/* — NOT SurfaceEventTypes), with appendHookInvoked/appendHookResult helpers so the invoked/result pairing and turn-enclosure stay consistent across bridges. appendHookResult also owns the durable record's semantics — the decision string (the hook's parsed decision, else 'stop' on continue:false, else 'pass') and the 500-character stderrSummary truncation derive from the HookOutput here, not per-bridge.

Per-dialect (the bridge plugins): building each event's stdin payload (CC's base+per-event field sets vs Codex's snake_case with turn_id/model extras), the dialect's env + ${CLAUDE_PLUGIN_ROOT} substitution (CC) vs none (Codex), and mapping the neutral HookOutput/MergedHookOutcome onto the harness's seam-specific typed Decisions (PreToolDecision, PromptDecision, ContinuationDecision, PostToolDecision).

Alternatives considered

One parameterized engine. Rejected because payload construction and decision mapping genuinely differ by dialect. Matchers, codecs, execution, merge rules, and events remain shared; each bridge keeps its payload and mapping explicit so its wire behavior is readable in place.

Consequences

Each bridge parses config, builds its dialect payload, invokes the shared runner and merge logic, maps the decision, and appends hook/*. Protocol tests cover every matcher mode, exit-code and codec field, runner plumbing, merge precedence, and audit helper at per-file 100%; bridge tests exercise the library's real load path. updatedInput is parsed but only logged and warned until the input-rewrite proposal lands.